1. Field of the Invention
The present invention concerns a method and a device for producing, from a series of N digital values representing the successive steps of a virtual digital waveform, and in a frequency band limited by a maximum frequency F.sub.M, a real analog waveform of which the variation of amplitude with time corresponds at least approximately to that which the virtual waveform, stepping at a given stepping frequency F.sub.d, would have.
Devices of this type are known in the domain of television broadcasting, and are used notably for the emission of auxiliary signals, such as signals for broadcasting data.
It is known in fact that television is used in many countries not only for transmitting audio-visual information corresponding to its prime function, but also digital information of collective interest; in France for example, the method known under the name "ANTIOPE" permits the broadcasting of digital data, such as stock market prices, accessible to any suitable information terminal connected to a television receiver.
In the case of broadcast data signals, the signals physically transmitted are analog waveforms intended to represent successive binary digital values, that is to say each consisting of a 0 bit or 1 bit.
It can thus be considered, in a slightly arbitrary but convenient manner, that the message to be transmitted, formed of N binary digital values, constitutes a virtual digital waveform which is, at its transmission, brought into existence as a real analog waveform.
While the first is defined only by the prescribed digital values, the second is defined by a group of physical parameters.
In particular, in the case of the generation of such broadcast data signals for television, the transmitter is conditioned by two parameters, which are on the one hand the stepping frequency F.sub.d of the virtual digital waveform to which the signal physically transmitted (that is to say the analog waveform) must correspond, and on the other hand by the maximum frequency F.sub.M present in the transmitted signal, that is to say the highest frequency of significant amplitude of the spectrum of the analog waveform.
2. Decription of Related Art
Traditionally, generators of broadcast data signals comprise means for re-reading, at a stepping frequency (which can be called a bit frequency) determined by an oscillator, and in the form of electrical signals of high level (for 1 bits) or low level (for 0 bits), the binary digital values of the broadcast data, and an analog low-pass filter for receiving these signals and producing an analog waveform of which the maximum frequency is limited to a value F.sub.M determined by the cut-off frequency of the filter.
However, although limited to a predetermined frequency band, the analog waveform transmitted must be recognised, at the receiver, as carrying the digital information constituted by the binary values which have served to form it.
For that, it is necessary, according to Shannon's theorem, that the maximum frequency F.sub.M of the analog waveform must be at least equal to twice the stepping frequency (or bit frequency) F.sub.d of the digital binary values.
As a result, in the known generators of broadcast data signals, in which the maximum frequency is determined once and for all by the physical characteristics of the analog output filter, the stepping frequency cannot be modified as desired, not only because it is itself physically determined by an oscillator, but also because the maximum frequency to which it is related cannot be changed without physical modification of the analog output filter.
These limitations constitute an obstacle to the necessity of being able to adapt to new norms and requirements and to transmit broadcast data signals of adjustable stepping frequencies.
It is known on the other hand that television broadcasting uses as auxiliary signals, in addition to broadcast digital data signals, test signals (or "test lines") intended for measuring the quality of the television transmission.
In fact, because of the worldwide dimension taken on at present by television transmission, international norms have been established for the control of the quality of transmissions.
This control is effected by transmitting, in addition to the audio-visual information intended for the public, predetermined test signals defined by norms and transmitted in a predetermined frequency band.
The observation, effected at the level of centralised television receivers, of the distortion suffered by the test signals received, permits an appreciation of the quality of the transmission.
At the present time, these test signals, of an analog nature, are produced by specific generators, different from the generators of broadcast data signals, which leads to the necessity of using a considerable amount of equipment for the generation of auxiliary television signals.
In this context, the object of the invention is to propose, for applications furthermore not limited to the emission of television test signals, means permitting the synthesizing, from digital values of a virtual waveform used at a stepping frequency F.sub.d, an analog waveform having a maximum frequency F.sub.M, while choosing the stepping frequency and the maximum frequency of this analog wave.
3. Summary of the Invention
To this end, the method of the invention, which when applied to television permits the synthesis of broadcast data signals as well as test signals, is characterised in that it comprises:
a preliminary sampling operation comprising producing digital input samples representative of the amplitude of a digital waveform in a number at least equal to the product M of the number N of digital values and the quotient of a predetermined conversion frequency F.sub.c and said stepping frequency F.sub.d, this quotient being greater than 1;
a digital filtering operation performed on these samples as a function of said choice of frequencies, and leading at least to the production of filtered digital words equal in number to said minimum number M, representing at least approximately, in at least two bits each, the amplitude of the analog waveform as a function of time; and
a conversion operation in which these digital words are, at said conversion frequency F.sub.c, converted into an signal forming at least approximately said analogue waveform, this conversion frequency F.sub.c being sufficiently high to be always at least equal to double said chosen maximum frequency F.sub.M.
Preferably, the filtered digital words contain a number of bits greater than the number of bits of the initial digital values.
In the case where said digital values are binary values, the conversion frequency F.sub.c is sufficiently high to be always at least equal to four times the chosen stepping frequency F.sub.d.
The filtering operation comprises generally a low-pass filtering, but can also comprise the application, to the digital samples, of a distortion necessary to compensate at least partially for the distortions introduced in the processing of the digital words by the conversion operation.
Advantageously, the number of digital samples produced before the filtering is greater than the number of digital words converted in the course of the conversion operation. More particularly, it can be envisaged that the number of digital samples should be a relatively high multiple of the number of digital words, for example equal to 8 times the number of digital words.
As the number of digital words is greater than the number of initial digital values, the number of samples is itself very much greater to this number of values.
This characteristic is important for the following reason: the analog waveform produced must not only have a chosen maximum frequency, but also represent as well as possible the initial virtual digital waveform. At the level of the digital filter, the shape of the virtual waveform is known only by the intermediary of the samples which are supplied to this filter. However, it is known that the shape of a sampled waveform is increasingly well known as the number of samples is increased. Thus, the waveform of which the shape corresponds to the digital words produced by the filter can better take account of the initial virtual digital waveform as the number of samples used for the filtering is increased.
The method of the invention can include an operation of storing the digital values and/or the samples in a memory, prior to the digital filtering operation and permitting this filtering to be carried out at a later time, the operation of storing the samples in a memory then replacing the operation of sampling in the manner described earlier.
Similarly, the method of the invention can include an operation of storing in memory filtered digital words, after the filtering operation and permitting the reproduction many times of the analog waveform from these words, while having performed said filtering only once.
Preferably, the method of the invention also comprises an operation of smoothing the analog signal resulting from the conversion, permitting the obtaining, from said analog signal, of said definitive analog waveform.
The device of the invention, intended to produce, from a series of N digital values representing successive steps of a virtual digital waveform, a real analog waveform of which the variation of amplitude with time corresponds at least approximately to that which the virtual waveform, stepping at a given stepping frequency F.sub.d, would have, is essentially characterised in that it comprises: sampling means for producing, at a sampling frequency K:F.sub.c greater than said stepping frequency, the digital samples representing in a supernumerary manner said digital values; a programmable digital filter adapted to receive these samples and to produce, at a frequency at least equal to a predetermined conversion frequency, digital words of at least two bits representing the variation of amplitude with time of the analog waveform, rendered capable of being synthesized in a frequency band limited by a maximum frequency F.sub.M at least equal to half said conversion frequency; and a digital-to-analog converter adapted to receive at least some of these digital words and to convert them at said predetermined conversion frequency into an analog signal forming at least approximately said analog waveform.
When the digital values to be transmitted are chosen in advance, the device preferably comprises a memory containing these values and programmable means for reading out these digital values at a chosen value of the stepping frequency F.sub.d.
To obtain a reproduction as faithful as possible of these digital values, the device preferably comprises sampling means which, for the reasons set out above, operates at a sampling frequency K:F.sub.c greater than the conversion frequency F.sub.c.
In the case where the digital filter produces at its output as many words as it receives input samples, that is to say in the case where it produces digital words at the sampling frequency, the device of the invention advantageously comprises at the output of the filter a buffer circuit supplying to the digital-to-analog converter, at the conversion frequency, digital words chosen in a regular manner from among the words produced by the filter.
Preferably, the digital-to-analog converter has a resolution, expressed in number of bits, higher than the number of bits of said digital values.
In a variant, the device of the invention can comprise a computer and a digital-to-analog converter, possibly followed by an analog smoothing filter.
To facilitate the implementation of the method of the invention, one can provide for a memory capable of receiving and storing the filtered digital words produced, for example at a later time, by the computer, and capable of reproducing them, at the real conversion frequency previously mentioned, to control the output digital-to-analog converter.
In this case, the computer can, instead of being used in real time, be used only for simulating, without time constraint for itself, the operation which a device of the kind described earlier would have when operating a sampling at the real frequency K:F.sub.c and a digital filtration in real time of the initial digital waveform; in this latter case, the digital words can be produced without the internal clock frequency of the computer playing a part in the implementation of the method of the invention; in particular, the use of a computer calculating the digital output words at a later time permits, with a computer of moderate performance, the calculation and storage in a memory of the digital words representing the amplitude of a waveform intended to be reproduced at a very high conversion frequency F.sub.c, this reproduction consisting essentially of supplying to the output digital-to-analog converter words read out of the memory at the conversion frequency F.sub.c.